Abstract Raman spectra of crystalline UO 2(NO 3) 2·2H 2O and its isotopic derivative UO 2 (NO 3) 2. 2D 2O have been investigated in the frequency range 20–3700 cm −1 from 300 to 90 K. An assignment of the bands in terms of UO 2 2+, NO − 3, and H 2O vibrations has been proposed in the internal and external mode regions. The temperature evolution of the Raman spectra indicates a structural phase transition in UO 2(NO 3) 2.2H 2O at 180 K. The transition has a large critical region and is associated with the strengthening of the hydrogen bonded network. The strengthening of hydrogen bonds distorts the geometries of entities bonded to it, which is mainly reflected in the temperature evolution of water stretching and librational modes. The changes in the crystal structure at 180 K are also reflected through the changes in the frequencies of the v 2 modes of nitrato (O-N-O 2) groups. Some lattice modes of the entities associated in the hydrogen bonded network also corroborate the changes in the crystal structure at 180K.